A fuel cell is an apparatus which directly converts the energy of a chemical reaction into low voltage direct electricity. The reactants employed in the chemical reaction include a fuel, such as hydrogen, and an oxidant, such as oxygen. When the fuel reacts with the oxidant, electrons are produced which results in the establishment of an electrical current.
Fuel cells can be broadly classified as being either primary fuel cells or secondary fuel cells. A primary fuel cell is a fuel cell in which the reactants used to produce electricity are consumed and must be replenished with fresh supplies of reactants in order to produce additional electricity. Primary fuel cells are relatively inexpensive to construct and simple to operate but have the disadvantage that a supply of consumable reactants must be maintained on hand for fueling the primary fuel cell whenever electricity is desired to be produced. Furthermore, primary fuel cells must be serviced on a regular basis to remove the reaction by-products, such as water. The need for maintaining a supply of reactants for fueling primary fuel cells and the need for periodic maintenance make primary fuel cells unsuitable for many applications and particularly unsuitable for use on satellites because of the added weight of the required supply of reactants.
Secondary fuel cells, hereinafter referred to as regeneratable fuel cells, operate in substantially the same manner as primary fuel cells in the discharge cycle. A fuel, such as hydrogen, reacts with an oxidant to produce water and electrons. However, in a regeneratable fuel cell the reactants which are consumed in the chemical reaction to produce electricity, are regenerated from the by-products of the chemical reaction and recycled for reuse in additional discharge cycles. To regenerate the reactants, energy from an external source is supplied to the fuel cells containing the reaction by-products. While various different forms of energy can be used to regenerate the reactants, specific attention will be directed to the use of electrical power to regenerate the reactants in a hydrogen fuel cell containing an aqueous alkaline electrolyte. To initiate the operation of a regeneratable hydrogen fuel cell, an initial supply of hydrogen is introduced into the fuel cell. When electrical power is required, the hydrogen is reacted with the hydroxy groups in the electrolyte which releases electrons to establish an electrical current and produce water as a reaction by-product. To this point, the regeneratable fuel cell operates in much the same manner as a primary fuel cell. However, when the regeneratable fuel cell is at least partially discharged, the spent reactants are regenerated by applying an electric current between the electrodes of the fuel cell. The electric current causes electrolysis of the by-product water, with the resulting regeneration of hydrogen for use in additional discharge cycles of the fuel cell.
Electrically regeneratable fuel cells are used in a variety of different applications. One especially important application for electrically regeneratable fuel cells is as auxiliary electric power sources for satellites. Satellites which are intended to be in orbit for extended periods of time are typically equipped with solar panels which include arrays of photovoltaic cells. When the solar panels are aligned with the sun, the solar energy emitted by the sun is converted by the photovoltaic cells into electricity. The electricity which is produced is used in the operation of the satellite and to charge the electrically regeneratable fuel cells. Thereafter, when the solar panels of the satellite are shielded from the sun, as, for example, when the satellite passes behind the earth, the electricity required to operate the satellite is obtained from the previously charged fuel cells.
Electrically regeneratable fuel cells are ideal for use on satellites as auxiliary electric power sources as they do not require the transportation of supplies of reactants on the satellite to fuel the cells as would be required with a primary fuel cell; do not require regular maintenance and can be charged and discharged for an indefinite number of cycles using electricity produced by the solar panels.
Problems are, however, encountered with conventional electrically regeneratable fuel cells in certain applications, and especially satellite applications. The fuel cell can produce only a relatively small amount of electricity in a given period of time. It is necessary in order to obtain significant amounts of electricity to use a large number of fuel cells which are electrically connected together to form a battery of fuel cells. A further problem is that the electrically regeneratable fuel cells are relatively heavy being made of metals such as nickel, silver, platinum and the like. Conventional batteries of regeneratable fuel cells also typically have a relatively limited electrical capacity and as such rapidly discharge in a relatively short period. This is a particularly serious problem in that a satellite can fail if the fuel cells become fully discharged in use before the solar panels reenter the sunlight and again produce electricity.
A further problem with the conventional electrically regeneratable fuel cells is that they have a relatively high internal resistance which can cause an internal drain of electricity within the fuel cell.
The weight and performance problems are further complicated because the batteries of the fuel cell used on satellites must be housed in gas tight casings to retain the regenerated hydrogen. The hydrogen within the casing must also be maintained at a high pressure for the fuel cell to operate properly, with pressure of 42 Kg/cm.sup.2 or higher commonly being required. The high pressures encountered require that the casings be made with thick walls of high strength steel alloys, further adding to the weight of the satellites.
What would be highly desirable would be an electrically regeneratable fuel cell which can be charged to higher voltage, has substantially greater capacity, reduced internal resistance and further, be lighter in weight than conventional electrically regeneratable fuel cells.